A sub-array of interferometers can be configured in a single fiber by utilizing a series of weakly reflecting fiber Bragg grating elements at a particular wavelength, along the fiber, spaced at an interval L. The fiber lengths between the gratings form interferometers, as light from a laser (at a wavelength matched to that of the gratings) passing down the fiber reflects off each grating. To isolate the interferometric effects from just one pair of gratings (i.e. one particular length L), the light coupled into the fiber can be pulse modulated. This produces a series of pulse returns from the array, as illustrated in FIG. 1. If the light from the array is fed to an interferometer, e.g. a Mach Zehnder, the path-difference between the arms of which is equal to the round-trip distance between two adjacent fiber Bragg grating reflectors in the array (2L), then a differential interferometer is formed. Such a path matching (compensating) interferometer is illustrated in FIG. 1.
Referring to FIG. 1, light which reflects off the first fiber Bragg grating reflector, but travels the long arm in the compensating interferometer travels the same optical distance as the light which reflects off the second fiber Bragg grating reflector, but passes through the shorter arm of the compensating interferometer. Referring also to FIG. 2, these optical components interfere at the output of the compensating interferometer, producing an interference signal the phase of which depends on phase perturbations in the fiber length L between the two fiber Bragg grating reflectors. This path matching occurs for any subsequent pair of fiber Bragg grating reflections along the array, resulting in a series of time-separated outputs from the compensating interferometer, each representing the phase a sequential length L of the fiber.
The system described above with respect to FIGS. 1 and 2 allows a sub-array of interferometers to be formed in a single fiber. However, such a system produces crosstalk due to the fact that optical pulses reflecting off one of the fiber Bragg grating elements in the array can undergo subsequent reflections at other fiber Bragg gratings and interfere with normally produced optical returns. This problem is illustrated in FIG. 3. Referring to FIG. 3, an array that comprises eight (8) fiber Bragg grating elements at a particular wavelength is illustrated, with each sensor element immediately adjacent to the next. Other topologies are possible and may be more desirable, but the fundamental source of crosstalk remains the same. A `primary optical return` from the array is defined as an output optical pulse generated from the input optical pulse which undergoes a single reflection from a fiber Bragg grating element. For a low reflectivity fiber Bragg grating, say 1%; the return signal is approximately 1% of the input source power. For the eight (8) fiber Bragg grating system shown, eight (8) primary pulses are generated (for seven (7) sensor lengths).
A pulse reflection off one fiber Bragg grating can also be reflected a second and third time off other fiber Bragg gratings and will produce a very weak return signal (down by R 3 on the input light level). This weak light signal exits the array in a time slot that would be associated with that of a primary optical return, and thus gives rise to crosstalk. The fact that the weak signal is interferometrically mixed with the stronger primary signal means that this source of crosstalk can be significant unless very weak grating reflectors are used. The number of secondary crosstalk pulses in the array increases strongly with the number of sensors multiplexed, and this may lead to unacceptably high crosstalk.
There therefore exists a need for an improved array topology for serial fiber Bragg grating interferometer arrays that reduces the crosstalk between the individual interferometers in the array.